The present invention relates to a projection unit for reflecting an image light, modulated by an image formation device, by a plurality of mirrors for performing enlarged projection.
U.S. Pat. No. 7,029,131 discloses a configuration of a projection unit for correcting the changes of a focal position (out of focus) due to a temperature rise (see FIG. 23). Specifically, a linear expansion coefficient of an inverted L-shaped holder 2, which is holding a mirror 1, is set to be greater than the linear expansion coefficient of a base 3 where the holder 2 is secured on the top portion 2a. By offsetting the expansion of the base 3 (arrow A) by thermal expansion with the displacement of the mirror 1 (arrow B) by thermal expansion of the holder 2, the distance C between the mirror 1 and the image formation device 4 is maintained to be constant even when temperature rises.
However, in the case of the configuration of U.S. Pat. No. 7,029,131, it is difficult to correct out of focus effectively. In order to correct out of focus, it is preferable that the linear expansion coefficient of the holder 2 is sufficiently greater than the linear expansion coefficient of the mirror 1. However, a material with a high linear expansion coefficient has low rigidity, so in terms of securing strength, a material with a true high linear expansion coefficient cannot be used as a material of the holder 2. As a result, with the configuration disclosed in U.S. Pat. No. 7,029,131, correcting out of focus when temperature rises is quantitatively insufficient, and out of focus inevitably remains.
In the case of the configuration disclosed in U.S. Pat. No. 7,029,131, in which the top portion 2a of the inverted L-shaped holder 2 is secured to the base 3, as mentioned above, the mirror 1 tends to become shaky due to vibration.
Also in the case of the configuration in U.S. Pat. No. 7,029,131 distance D from the position, where the holder 2 is secured to the base 3, to the center axis CL of the mirror 1, is relatively long, so the displacement of mirror 1 tends to occur in a vertical direction (direction perpendicular to the center axis CL of the mirror 1) when temperature rises.